I travel constantly for work, basically live out of my suitcase, so location is not a big factor. Not dogmatic about Mexico, but I figured...
1. It's close to the US which is good just in case I need to come back quickly or temporarily at any time
2. It's cheap and easy to travel to, drivable to all border cities
3. The camp itself would be much cheaper than one in the US
4. I could learn Spanish via immersion while I'm there (I've always wanted to learn Spanish)

I suppose I'd consider an all boxing camp, although MMA/kickboxing/muay thai would be preferred. Weight loss to achieve elusive six pack is another purpose for what I seek (I need to lose another 12-15lbs or so to be ripped).

If you're deadset out of the country tristar is in Montreal, you could learn French. They have dorms, petty strict rules but great pro practices and world class fighters. I was there last year and it was amazing. That's Canada, so slightly cheaper and cost of living is about on par with any city.

Are you going just to train and get fit or are you preparing for a fight? Because when you say training camp I'm thinking you're preparing for a fight, not just trying to get fit. Which is completely fine, just your options maybe different because living in fighter dorms is pretty much reserved for guys in camp.

Something to consider as well is southern California is aka north Mexico. You'll run into just as much Spanish as you would in a tourist area in Mexico or near the border.